Various forms of information display systems have been proposed in the past for the purpose of indicating the operating conditions and the internal states of the vehicle. For instance, Japanese patent publication No. 51-20787 proposed a system in the form of a centralized warning system. According to this proposal, a number of check points are selected in advance and any abnormal states found at these check points are displayed as warning signals according to a certain hierarchy of priority scaled to the degree of importance. The display unit for this display system is so small that it can therefore show only a limited amount of information at any given moment. As a matter of fact, this is generally the case with display systems of this kind. Therefore, if more than one abnormal condition has occurred, the system is capable of indicating only one of the abnormal conditions and all the necessary measures may not be taken at a sufficiently early stage.
To the end of eliminating this problem, Japanese patent laid open publication No. 53-87694 discloses an information display system which is provided with means for indicating that there are a plurality of items to be displayed by giving a display literally saying "plural". However, this system can not display the content of any one of the plural additional abnormal conditions and cannot indicate how many items are detected to be in abnormal conditions and are required to be displayed.
Beside from this problem, as the number of items of information to be displayed increases, the handling of the information display system becomes less favorable and the convenience of the system tends to be diminished. It is therefore desired to have an information display system which occupies little space by having a small display area and a limited number of switches and yet can display a comprehensive range of information without requiring any complicated handling on the part of the user.
The quantity of fuel remaining in the tank and the distance which can be covered with the fuel remaining in the tank are important items of information which are desired to be displayed as trip function information. Japanese patent laid open publication No. 57-12322 discloses such a method of computing the distance that can be covered with the fuel remaining in the tank. According to this method, the distance is displayed only when the quantity of the fuel remaining in the tank has agreed with one of a plurality of key values. Japanese patent laid open publication No. 56-112336 proposes a method and apparatus for issuing a warning with synthesized speech when the quantity of the fuel remaining in the tank has reached one of two values and using different warnings for the two different values.
These proposals have the disadvantage that the display is intermittent and does not provide a continual display of the trip information. Furthermore, the computation of the distance that can be covered by the fuel remaining in the tank tends to be inaccurate because the detection of the quantity of the fuel in the fuel tank involves a greater error as the quantity decreases. This is particularly significant when the fuel quantity is estimated from the cumulative flow rate of fuel injection in the engine because the error accumulates as the fuel level drops.
Most items of the trip information are based on the cumulative mileage of the vehicle. Therefore, if the cumulative mileage stored in the memory of the control unit is erased due to power outage, repair or other servicing, accurate management of the vehicle is not possible any more. Therefore, it is desired to have means for reinstalling the cumulative mileage when it is erased from the memory without sacrificing the simplicity of the handling of the information display system.
Trip information related to the gas mileage of the vehicle requires an indication of the filling the fuel tank. A float level meter is typically equipped to the tank but is unsuitable for accurately detecting the full level of the fuel tank because it tends to indicate the full level of the fuel tank whenever the fuel level is close to the actual full level. Japanese patent publication No. 58-38726 proposes a device for displaying the distance that can be covered by the vehicle and the quantity of the fuel remaining in the tank. According to this proposal, the driver turns on a certain manual switch when the fuel tank is filled up. The distance that can be covered by the vehicle with the remaining fuel is computed according to a certain average fuel mileage when the quantity of the fuel remaining in the tank is large and this fuel mileage is updated with current fuel mileage as the quantity of the fuel in the tank gets less.
However, if one forgets to turn on the tank full switch, the information display system cannot display any information related to fuel consumption until the vehicle makes a next stop to fill up the fuel tank. It may be possible to assume that the tank has been filled up if the ignition switch is turned on when the float sensor indicates a substantially full fuel level in the tank. However, since a tank full state is assumed every time the ignition switch is turned on with a substantially full tank, the accuracy of the display based on this information may not be sufficiently accurate.